To address growing recognition that many couples attempting pregnancy are unsuccessful, a prospective pregnancy study was conducted to identify determinants of male and female fecundity and fertility. Previous studies involving experimental animals and wildlife populations report a spectrum of adverse reproductive and developmental effects associated with exposure to environmental hormonally active agents (HAAs). Virtually no population based epidemiologic studies have addressed this issue, despite increasing public concern about the effect of chemicals and environmental contaminants on human reproduction and, thereby, serves as the impetus for study. In addition, several lifestyle behaviors alleged to affect human reproduction were studied as well. This will be the first study in the US to use a couple-based approach to study determinants of fecundity and fertility and, specifically, to assess the interplay between lifestyle and HAAs. It will be the first to estimate the incidence of infertility as only prevalence data currently exist. Successful completion of this study will help delineate mechanisms that underlie impaired human fecundity and fertility. A cohort of 501 couples desiring pregnancy were recruited and prospectively followed for 12 at- risk menstrual cycles to determine whether HAAs and lifestyle factors affect the probability of conception, early pregnancy loss, and infertility. Couples completed baseline interviews and provided approximately 20 ml of blood for the analysis of HAAs and metabolites of cigarette smoke, caffeine, and dietary supplements. Male partners provided semen samples for an objective assessment of male fecundity. Couples use journals to record daily use of cigarettes, alcohol, caffeine, and dietary supplements as well as physical activity, life stressors and intercourse. Women were instructed in the use of home fertility monitors and pregnancy kits to determine ovulation (screening for estradiol and LH) and hCG-confirmed pregnancies. The data are currently being analyzed, given that follow up of couples is now complete. This is the first study in the US to use a couple-based approach to study determinants of fecundity and fertility and, specifically, to assess the interplay between lifestyle and hormonally active agents in the environment. The ensuing data will be directly relevant to regulatory agencies charged with classifying environmental agents as to reproductive and developmental toxicity, and public health agencies that respond to growing public concern about the effects of environmental chemicals on pregnancy outcomes and reproductive health. This study is also the first to address the effect of dietary supplements on reproductive and developmental processes and whether the effect is maternally, paternally or parentally mediated. The study is directly responsive to the strategic framework of NICHD by addressing preconception, periconception and in utero influences on human reproduction and development. It is responsive to several of NICHD[unreadable]s goals (viz., sequelae of reproductive health, increasing the probability of good birth outcomes, and helping to delineate the developmental origins of health disparities).